Polypropylenes include isotactic polypropylenes and syndiotactic polypropylenes. The isotactic polypropylenes give films that are inexpensive and possess high transparency, toughness, humidity resistance and heat resistance. Such films are therefore widely used as packaging materials. Of the isotactic polypropylene films, ethylene/propylene random copolymer films are particularly excellent in transparency but reduce the transparency and flexibility with increasing film thickness. For example, the film thickness is up to about 60 μm in order to obtain sufficient transparency such that the packaging material does not deteriorate the appearance of content. Accordingly, production of thick polypropylene films high in transparency and flexibility has been difficult.
On the other hand, the syndiotactic polypropylenes are known to be obtained by low temperature polymerization in the presence of a catalyst that includes a vanadium compound, an ether and an organoaluminum. The polymers obtained by this method, however, possess low syndiotacticity and cannot exhibit the inherent syndiotactic properties.
In this connection, J. A. Ewen et al. were the first to find that high-tacticity polypropylenes having a syndiotactic pentad ratio of above 0.7 can be obtained by polymerization in the presence of a catalyst containing a transition metal catalyst with asymmetric ligands and an aluminoxane (J. Am. Chem. Soc., 1988, 110, 6255-6256).
The polymers obtained by the method of J. A. Ewen et al. have high syndiotacticity and are more elastic than the isotactic polypropylenes. However, such flexible forming materials cannot satisfy the flexibility, rubber elasticity and mechanical strength as required in the field where soft vinyl chloride and vulcanized rubbers are used.
Attempts have been widely made to improve the polypropylene's flexibility and impact resistance by incorporating an ethylene/propylene copolymer rubber or the like. Articles from the resin compositions obtained by this method show a certain level of flexibility and impact resistance, but the rubber elasticity and mechanical strength thereof are insufficient.
Medical tubes include tubes for introducing or deriving a substance into or from a body, and catheters inserted into the body for test or treatment. Specific examples of the medical tubes include catheters such as urinary catheters, stomach catheters and suction catheters, tubes such as infusion solution tubes, enteral feeding tubes, peritoneal dialysis tubes, blood transfusion tubes and tubes connected to a urinary catheter to guide urine to a urine collection bag, circuit tubes used in blood circuits for hemodialysis, artificial heart lungs and plasmapheresis, and tubes for transporting substances in the medical field. The transporting tubes for medical substances include tubes attached to multiple blood bags and tubes connecting an aspirator and a catheter. Many of the conventional medical tubes are made from polyvinyl chloride that is inexpensive and possesses excellent kink resistance and a certain level of flexibility (pliancy). However, alternative materials have been required out of consideration to the environment and the like.
The alternatives studied so far include styrene elastomer compositions (JP-A-2000-63577, JP-A-2001-252348 and JP-A-2001-1432), thermoplastic polyurethane compositions (JP-A-H05-84293), and syndiotactic 1,2-polybutadiene compositions (JP-A-2000-334038 and JP-A-2001-104473). The fact, however, is that these compositions have low versatility and practical utility due to insufficient flexibility and high costs.
To achieve the versatility and practical utility, studies have been made on copolymers of ethylene and α-olefins of 3 or more carbon atoms, and acid copolymers of ethylene and vinyl acetate. However, none has satisfied performances required such as flexibility, heat resistance and kink resistance.
Of the polypropylenes such as the isotactic polypropylenes and syndiotactic polypropylenes, the isotactic polypropylenes are inexpensive and excellent in transparency and heat resistance, and are therefore widely used in various packaging materials and industrial materials. However, their flexibility is unsatisfactory. To solve this problem, compositions have been studied in which a flexible material such as an ethylenic elastomer is blended with polypropylene. However, none has satisfied performances as required.